Treatment of a B cell lymphoma with E. coli LPS resulted in an increase in surface IgM (sIgM but not Ia(d) nor H-2(d). Treatment with beta interferon increased expression of Ia(d) and H-2(d), but not sIgM. A B cell lymphoma was drug marked and B cell hybridomas obtained. Some of the hybrids secrete IgM. Others express IgM and H-2(b) from the strain of mouse, C57BL/6 used for fusion.